1. Field of the Invention
This invention relates to an improved fluid distribution system for continuously distributing hot fluid evenly across the top face of a fill assembly in a cross-flow water cooling tower. Specifically, this invention provides an uniform fluid head to the distribution pan and provides an in-line basket filter to prevent clogging of the metering nozzles in the pan.
2. Description of the Prior Art
Evaporative water cooling towers are well known in the art and generally involve pumping a hot heat exchange liquid (usually water) up to and evenly across an open distribution basin at the top of the tower wherein a heat exchange or fill media gravitationally receives the hot water descending through basin nozzles, in an intimate, evaporative, heat exchange relationship with a stream of cool air passing horizontally through the fill assembly. The cooled water is ultimately collected in an underlying sump and then returned to an offsite process which continuously uses the cooled water in a heat exchange type of application. Cooling towers of the type described may employ natural draft techniques for creating the cross-flow movement of cooling air through the fill assembly such as by utilization of a hyperbolic stack or they may employ a mechanical device such as a common propeller fan or a forced or induced draft centrifugal fan. In any event, when the air contacts the water, heat and mass transfer occur simultaneously, resulting in a small portion of the hot water being evaporated into the air. The energy which causes the water to evaporate is supplied from the release of sensible heat from the hot water. Accordingly, as the sensible heat is liberated from the hot water, the water temperature is reduced and cooling is accomplished. The air stream is used for sweeping away the evaporated water, and is exhausted from the tower as a moist, warm, air stream.
It is the functional objective of conventional cooling apparatus distribution basins to receive the incoming stream of hot water and direct the same toward a plurality of spaced metering nozzles located in the bottom of the basin for uniform discharge across the top face of the underlying fill media. Depending upon the required cooling capacity, water flow rates into the apparatus distribution basin can be on the order of 200 to 1600 gallons per minute or more. With the larger capacity requirements, the upper face of the fill media may be of such dimensional size that some of the more remote metering nozzles are located a significant distance from the hot water inlet supply pipe, making uniform water distribution difficult.
The amount of water which passes through each of the nozzles depends upon the size and type of nozzle, as well as the head pressure of water above the nozzle. Cooling towers typically provide metering nozzles that are equivalent in diameter and are spaced at precise intervals. As a result, the major variable affecting the rate of water flow through the nozzles is the amount of water head pressure above each of the nozzles. Accordingly, it is critical to provide a uniform water head pressure above each of the nozzles throughout the entire area comprising the distribution basin.
Moreover, when incoming flows are applied to an open distribution basin, a resultant steady state water depth is typically found where the water level is shallower near the inlet supply pipe than at the opposite end of the basin. To some extent, this problem is caused by flow turbulence, which causes unsteady water levels throughout the basin such that the static head to each nozzle becomes variable. Under these conditions, the nozzle discharge flow rate can be uneven, if not substantially unpredictable. Most often though, as when the water flow rates approach maximum levels, the total energy of the water can cause the water to "shear" across the tops of the nozzles closest to the inlet supply pipe and not allow the water to downwardly descend into the nozzles. More specifically, this condition exits due to the total energy actually consisting of a velocity energy component and a static energy component, with the velocity energy component being very large near the inlet supply. Such a condition will cause a reduced flow of water through the nozzles closest to the supply pipe even though sufficient water head exists.
In an effort to distribute the incoming hot water to all of the metering nozzles uniformly, various methods and devices have been devised. One such method for providing uniform water distribution to a cross-flow cooling tower is described in U.S. Pat. No. 4,579,692, and involves feeding the hot water into the distribution basin from an overhead supply pipe which is central to the basin. That distribution system utilizes a stilling chamber and a flume contained within the chamber to receive water above the distribution pan. One end of the stilling chamber is connected to the overhead supply pipe and the other end is connected to the flume at its center, thereby providing separate flume sections which extend longitudinally from the center of the basin to each basin, or tower edge. As the hot water flows into the stilling chamber and reverses direction, it loses its velocity energy before flowing into the flume, where it overflows the sidewalls and uniformity distributes across the basin.
Another method for providing uniform water distribution to a cooling tower basin is described in U.S. Pat. No. 5,180,528, where the hot water supply pipe is brought in from the bottom of the distribution basin at one corner of the tower. The distribution system described in this patent utilizes an inlet chamber that is connected to a side of a flume co-extending the longitudinal length of the distribution pan. An internal flume weir wall co-extends the length of the flume. As the hot water enters the chamber from the bottom, it changes direction by 90.degree. and loses its velocity energy before it enters the flume. Once inside the flume, it travels the longitudinal length of the basin, eventually overflowing the internal weir walls. Baffles attached to the basin then direct the water to the nozzles.
Another arrangement utilizes a centrally located, and top feeding pre-distribution box internally containing a flow proportioning valve and a system of weir plates for evenly distributing the hot water throughout the box for eventual distribution in the basin. This arrangement is described in U.S. Pat. No. 4,592,878.
Most of the top feed arrangements have either utilized a centrally disposed pre-distribution box or a flume, located at the distribution pan back side, for changing the direction of flow by 90.degree. as a means for equalizing the flow in all directions.
All of the methods described above have been successful in providing even water distribution to a distribution basin where the hot water to be cooled is supplied from either above or below the basin, but none of the above methods consistently provide a low pressure loss arrangement across a wide range of turndown ratios as does the present invention. Furthermore, the present invention even includes an in-line strainer or filter within the pre-distribution box for ensuring continued uniform distribution by removing system debris which can lead to nozzle clogging and uneven water distribution. The strainer of the present invention is constructed such that it does not effect the pressure of the water as it flows through the pre-distribution box.
In any of the above mentioned systems, it is possible to add an in-line filter in the system piping. However, even with the simplest filter, one which spans the piping diameter, the pressure losses can be significant due to the reduction in the effective cross-sectional area. Most in-line filters of this type reduce the open cross-sectional area to about fifty percent, thereby forcing the water velocity to increase to approximately twice the original velocity with a significant pressure drop. When filters of this type begin to clog, the open area of the filter is reduced even further, creating even greater pressure loesses that cause the water flow to be reduced and possibly result in total shutdown of the system because the supply pump cannot overcome the additional pressure losses. As an example, a design pipe velocity of 8 ft/sec with a 50% open filter would have additional pressure loss of approximately 1.7 feet of water. If the filter clogged to only 25% open, the pressure loss would increase to approximately 6.8 feet of water. If it clogged to only 10% open, the pressure loss would increase to 42.5 feet, which happens to be 40.8 feet more than the base clean system! There is almost no cooling apparatus piping system that can handle this additional loss. Most systems are designed with only minimal reserve of balancing valves and anticipated pipe future scaling. The most common result would be that the maximum shut-off pressure of the pump would be exceeded, and no water would flow through the system, thereby shutting down the whole cooling system.
An alternative to an in-line filter arrangement is to provide a larger, basket-type strainer as a means for lowering the initial pressure drop across the strainer, as well as for lowering the total pressure losses as the strainer becomes clogged. This situation is obviously an undesirable choice in that it requires larger, more costly equipment and the room to include it in the system. Consequently, a high percentage of the water distribution systems do not contain either of such filters. As a result, many distribution systems ultimately experience uneven water distribution due to pipe scale and other debris clogging the nozzles. Furthermore, when the apparatus is operating, it is practically impossible to clean an in-line filter or basket strainer without shutting down the system.